The external panels and/or flight control surfaces of in ground support aircraft and military aircraft are exposed to various types of damage ranging from impact to ballistic type damage. This damage can result in stress concentration fractures, which, if not repaired, will propagate and grow with use. Typical prior art methods of repairing such damaged areas in aircraft having metal skin include the step of positioning a metal patch on the exterior of the damaged area so as to overlay the damaged area and overlap the aircraft skin. The patch is then attached to the skin with fasteners and caulked to prevent leakage. This method is described in U.S. Pat. No. 4,517,038, to Miller.
It is desirable to use a patch made from the same material as the aircraft skin. Thus, a metal patch is used for a metal-skinned aircraft and a composite patch should be used for a composite-skinned aircraft. However, metallic and composite materials have different physical properties which result in loads being transferred through the materials and patches differently. Metal is isotropic and transfers loads in all directions with equal facility. Composite structures, however, are typically designed to be anisotropic and thus distribute loads well in the direction of the embedded fibrous reinforcing material while transferring loads in other directions less effectively. Therefore, it is extremely important when patching composite materials that the patch be capable of transferring both compressive and tensile loads across the damaged area in a manner so as to cooperate with the isotropic properties of the material to be patched.
In addition to the above, it is highly desirable for both aerodynamic efficiency and the reduction of radar cross section (RCS) that the patch be flush with the adjacent sheet of material. The metallic airplane patch disclosed in Miller is not flush. In addition, a truly flush patch is desirable because the junction between the patch and the material is not affected as much as a nonflush patch by aerodynamic loading. This aerodynamic loading can cause the bond between the patch and the material to loosen such that water and other external elements could enter the junction therebetween to the detriment of the bond.
Difficulty has been encountered in providing a blind-side flush patch in which access to the damaged area is only available from the outside. Such a repair is very difficult in the field because the aircraft component (e.g., the wing section) cannot be disassembled for access to the interior of the wing. Therefore, a need exists for a field repair method and apparatus for forming a structural blind patch in a sheet of composite material.